Apple has historically helped Mac users to upgrade to a new machine via FireWire Target Disk Mode, which converts their old Mac into a hard drive that their new machine can import data, files and settings from via Migration Assistant.
However, as some new Mac models have dropped FireWire, Apple added a network copy feature to Migration Assistant that enabled a new machine to connect to a previous Mac via a network connection to perform the same task.
In Lion, Migration Assistant offers to connect to a PC over the network as well, enabling new Mac users to pull their data from an existing Windows-based computer.
It's not clear if the Assistant requires client software running on the Windows PC to work; performing a Mac to Mac network migration simply uses the same app on both ends, one sharing to, the other copying from. The developer reporting the new feature did not note if anything was required on the PC side.
Update: A developer has noted that Apple supplies a MigrationAssistantSetup.exe program for PC users on its developer site, which acts as a client for the Mac Migration Assistant to pull data from as it performs a migration.